Process for electrowinning titanium from lower valent titanium alkali chlorides



Oct. 3, 1961 BR ET AL 3,002,905

PROCESS FOR ELECTROWINNING TITANIUM FROM LOWER VALENT TITANIUM ALKALICHLORIDES Filed May 27, 1955 E2 or C (Anode) L gage Cai'ho/yfe INVENTORS, Abner B r arm er 3,002,905 PROCESS FOR ELECTRUWINNlNG TTTANIUMFROM LOWER VALENT TITANKUM AEKALE CHLORIDES Abner Brenner, Chevy Chase,Md, and Joseph M. Sherfey, Arlington, Va, assignors to the United Statesof America as represented by the @ecretary of the Filed May 27', 1955,Ser. No. 511,319 1 Claim. (@l. %64) (Granted under Title 35, US. Code(1.952), see. 2st) The invention described herein may be manufacturedand used by or for the Government for governmental purposes without thepayment to use of any royalty thereon.

This invention relates to a process for electron inning titanium from atrivalent compound dissolved in a fused bath of alkali halides.

, Because of the widespread interest in titanium at the present time theproduction of the metal by electrolysis has received much attention.Several electrolytic processes have been developed and the process whichwill prove to be the most satisfactory for commercial production oftitanium will depend on the convenience and economy in producing theinitial source of titanium, the readiness with which the metal can beisolated from the fusedsalt bath and the purity of the metal obtained.Prior art electrochemical procedures for depositing titanium from fusedsalt baths are found in Japanese Patent No. 3,859 (1952); British PatentNos. 678,807 and 682,919 (1952); Australian Patent No. 256,951 (1951);and Argentine Patent No. 81,510 (1951). These patents deal with thepassage of TiCl, vapor over a cathode in mersed in a fused salt bath.The resulting solution may contain titanium in either the divalent ortrivalent state. In the British Patent No. 698,151 hydrogen is passedwith the TiCl, vapor into the bath to effect the reduction. A fewpublications have also appeared dealing With the deposition of titaniumfrom fused halide melts into which the titanium has been introduced inthe form of titanium trichloride, TiCl for which see: G. D. P. Cordnerand A. W. Worner, Australian J. Applied Science, vol. 2, page 358(1951); Shinzo Okada, Makoto Kawane and Mitsunao Takahashi, Bull. Eng.Research Inst, Kyoto Univ., vol. 6, page 57 (1954); J. Chem. Soc. Japan,Ind. Chem. Soc, vol. 56, page 410 (1953); A. Brenner and S. Senderoif,J. Electrochem. Soc., vol. 99, page 2230 (1952). However, this method ofintroducing titanium does not prove commercially feasible.

'Three recent US. patents, Nos. 2,707,168, 2,707,169 and 2,707,170 dealwith the electrodeposition of titanium using titanium monoxide as thesource of titanium.

Titanium metal in commercial quantities is now being produced solely bythe Kroll process, an inherently expensive batch operation involving thechemical reduction of titanium tetrachloride with metallic magnesium.Because of the high cost of this'method there is much current interestin the development ofan economically feasible process for electrowinningtitanium metal.

When conducted on a laboratory scale this is easily accomplished. A widevariety of titanium compounds yield a deposit of metallic titanium whenelectrolyzed in any one of the many possible molten salt baths. Theseprocesses are beset with many difficulties, both technical and economic,which prevent their utilization on a commercial scale.

One such process that has been extensively investigated 'utilizes a bathconsisting of potassium fluotitanate (K TiF dissolved in a molten saltbath, such as a eutectic mixture of sodium chloride (NaCl) and potassiumchloride (KCl). From a commercial standpoint this system has manyobjectionable features. In the first are a Patented @ct. 3, 1961 placethe solute (KgTiFs) is prepared from an aqueous solution, resulting in aproduct which is diflicult to render anhydrous. Moreover this bathoperates well only at a relatively high temperature (800900 C.) and highcurrent density (200500 amp./dm. The fluoride content of this bathconstantly increases, necessitating its periodic disposal.

One of the most serious objections to this bath is the high cost of thesolute K TiF This is caused by the difiiculties encountered in preparingit in a pure state and the relatively high cost of the hydrofluoric acidused in its preparation.

The obvious solution to these last difliculties lies in the use oftitanium tetrachloride or compounds derived from it in place of K TiFTitanium tetrachloride is comparatively cheap and is being produced in avery pure state on a commercial scale. It is an unfortunate fact,however, that this compound is a volatile liquid which is not retainedin an adequate concentration even in a lowmelting solvent such aspotassiumdithium chloride eutectic (melting point 350 C.). Thedifficulty resulting from the volatility of titanium tetrachloride canbe avoided by use of either of the lower valent titanium chlorides,titanium trichloride (TiCl or titanium dichloride (TiCl neither of whichis volatile. Both of these compounds have been used in fused melts toelectrodeposit titanium on a laboratory scale and offer many advantages.Titanium trichloride, for example, is readily soluble in the lowtemperature potassium chloride-lithium chloride eutectic bath and yieldstitanium metal with a high current efficiency.

An object of the present invention therefore is to disclose a processfor electrowinning titanium from a trivalent compound dissolved in afused vath of alkali or alkali earth halides which involves twoessential conditions: (1) the solute in the fused bath is a trivalentcompound of titanium introduced into the bath in solid form and havingthe approximate composition corresponding to the formula NaTiCl (2) theuse of a glass diaphragm to enclose the cathode chamber to preventmixing of the catholyte and the anolyte. I

The use of either of the two simple lower valent chlorides of titanium,in view of their low volatility and good solubility, has obviousadvantages, but to develop a commercially feasible process based onthem, two problems must be overcome which had not been solved prior toour work. In the first place there is no known method for producing themin anhydrous condition on a commercial sca e. In fact, they arelaboratory curiosities. See Apparatus for the Preparation of AnhydrousTi tanium Ill Chloride and Titanium III Bromide, Journal of Research ofthe National Bureau of Standards, vol. 46, No. 4, April 1951, J. M.Sherfey.

The second problem which would be common to any electrolytic processbased on a reduced titanium compound solute, is the necessity ofpreventing anodic oxidation of the solute. Without such prevention thecurrent efiiciency would be greatly lowered and, in the case of achloride bath; volatile TiC1 would be formed and lost from the bath.

The use of diaphragms to effect separation of the catholyte and anolyteis a common commercial practice in electrodeposition from aqueoussolutions. However, the successful use of such a diaphragm in aproduction scale molten salt bath has never been reported in theliterature. Few materials can withstand the conditions which exist insuch baths. Aluminum oxide has some utility, but porous diaphragmscomposed of this material permit serious mixing of catholytc and anolyteif the pores are large and have too high an electrical resistance if thepores are small. Furthermore, although porous diaphragms decrease mixingresulting from convection they do not affect mixing caused by electricalmigration of ions.

It can thus be seen that an electrowinning process which would utilizean inexpensive and ea ily prepared lower valent titanium chloride as asolute. in a cell having a satisfactory diaphragm would have manyobvious advantages over the Kroll process. In the prior art patentsdiscussed above no suitable diaphragm was shown to have been developed.FIG. 1 of Patent. No. 2,707,168 shows a graphite barrier 20 and in FIG.2 a ceramic diapbragm, 22, of zircon or mullite having a porosity of 20percent is shown, neither of which provides complete separation of anodeand cathode compartments.

A diaphragm which prevents mixing by electrical mi gration is proposedherein. The solutes are described in our copending application SerialNo. 310,147,;filed on September 17, 1952 (now Patent 2,765,270). Brieflyit consists of any one of a. series of stable reduced. alkali titaniumhalide complexes produced by the.- reaction of an alkali metal with atetrahalide of titanium. The composition of one such compoundapproximates that represented by the formula NaTiCh.

To prevent oxidation of the compound the cell must be equipped with aneflicient diaphragm. It has been demonstrated by the applicants (seeour. copending application Serial No. 448,396, filed on August 30, 1954)and others (see Ingeberg, U.S. Patent No. 1,299,947), that glass is anideal material for construction of such a diaphragm as it efiectsabsolute separation of the catholyte and anolyte except for the passageof a positive ion, ordinarily sodium, from the anolyte through the glassmembrane and into the eatholyte. Another obvious advantage stems fromthe fact that a glass diaphragm simplifies. the protection of the cellfrom contamination by atmospheric gases. Such protection is alwaysnecessary when electrodepositing titanium from a high temperature bath.However, when a glass diaphragm is used it is only the cathodecompartment that needs protection. The entire anode compartment can beexposed to the air. The proposed cell alfords considerable flexibilityin the. choice of an anolyte melt in that it must meet only tworequirements. First, the maximum permissible melting point is thattemperature above which the diaphragm is. too soft to have adequatemechanical strength. The glass diaphragm may take the form of asemi-molten, horizontal layer in the cell, in which case the meltingpoint maybe higher. Second, the cation in the anolyte must be the sameas the current carrying cation in the glass. Unless thi is the case,electrolysis will change the composition of the glass. Thus, a sodiumglass diaphragm would require a sodium salt anolyte. A process employingthese principles has been operated successfully on a laboratory scale.The catholyte consisted of KCl-LiCl eutectic containing about percent byWeight of NaTiCl contained in a Pyrex glass vessel that also served as adiaphragm. The top of the vessel was fitted with an air-tight lidthrough which passed a tungsten rod which served as cathode. Thev top ofthe vessel was also provided with an airlock which permitted removal andreplacement of the cathode while an inert atmosphere was maintained inthe catholyte chamber.

The lower portion of the glass vessel containing the catholyte wasimmersed in an open vessel containing the. anolyte which was a lowmelting mixture of molybdic oxide (M06 and sodium molyhdate (Na M OCarbon or iron was used as an anode.

This cell was operated at a temperature somewhat below the softeningpoint of the Pyrex diaphragm (550- 600 (2.). The current density wasabout 2.0-20.0 anr1p./dm. although a considerable variation of thisvalue is permissible. The cathode current efiiciency based on trivalenttitanium was in excess of 90 percent. Essentially all of the titaniumintroduced as a solute was recovered in the form of metallic crystals orpowder.

The following two variations of the process were demonstrated on atest-tube scale: (1) a divalent sodium titanium chloride can be used asa solute. Its preparation and the preparation of NaTiCl are similarexcept that the reactants, sodium and titanium tetrachloride, are in themolar ratio of two to one respectively. The product of this reactionforms a solution containing divalent titanium ions when dissolved in themolten salt melt. (2) Metallic molybdenum was deposited from a'cellwhich was identical to the cell described above for the electrodeposition of titanium excepting the substitution of a trivalentmolybdenum complex (K Mocl for the trivalent titanium complex (NaTiCh).

A typical example of the operating of our process follows: 10 grams ofsodium titanium halide, NaTiCl' and 40 grams of potassiumchloride-lithium chloride eutectic were placed in a Pyrex test tubeprovided with a rubber stopper having three holes. A nickel or tungstenrod was passed through the center hole to serve as the cathode. Throughthe other two holes tubes were passed for flushing the cell with argon.This test tube assembly comprised the cathode compartment. The bottompart of the tube containing the mixed salts was placed in a molten bathconsisting of sodium molybdate and molybdenum trioxide at a temperatureof 575 C. A current of about 1. ampere was passed between the rodserving as cathode and an iron rod placed in the outer vessel of fusedmolybdate to serve as anode. After about 10 percent more than thetheoretical amount of current had passed, the contents of the test tubewere extracted with water and the titanium collected on a filter as apowder. The yield of titanium was over 90 percent.

An apparatus in which the above processes may be carried out isillustrated in the accompanying drawing.

it should be. clearly understood, however, that the above illustrationis solely by way of example, and is not to be construed as a limitationupon the spirit or scope of the appended claim.

This case is a continuation-in-part of our application Serial No.448,396, filed on August 6, 1954 (now abandoned).

We claim as our invention:

The process of electrowinning titanium from lower valent titanium alkalichlorides contained in the catholyte of an electrolytic cell in whichthe anolyte consists of a mixture of M00 and Na MoO which processcomprisesmaintaining a separation of said anolyte from said catholyte bymeans of a solid glass diaphragm impervious to the passage of thecatholyte or anolyte, passing an electric current through said cell andsaid diaphragm and recovermg titanium as a powder.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Cordner et al.: Australian Journal of Applied Science," vol.2, September 1951, pp. 358-367.

